Grey cast iron contains carbon in the form of "flakes" and has low tensile strength and a low modulus of elasticity, but relatively high thermal conductivity. Nodular irons or ductile irons contain carbon in the form of "nodules" or "spheroids". Nodular iron has higher strength and good ductility, but lower thermal conductivity. Compacted graphite irons contain graphite in the form of flakes having rounded ends and generally the flakes are short and thick. Actually, compacted graphite iron is intermediate nodular iron and grey iron and has generally intermediate properties. The strength and ductility of compacted graphite iron approaches that of ductile, yet the thermal conductivity is nearer to that of grey iron.
In both ductile iron and compacted graphite iron, the modified graphitic structure is produced by the inoculation of essentially white cast iron compositions with a "nodularizing inoculant". Commercially, the "nodularizing inoculants" are magnesium or cerium or a mixture of the two. Other known nodularizing inoculants include calcium, lithium, sodium, barium, and the rare earth metals. In the case of compacted graphitic iron, titanium is usually added to inhibit nodularization and to insure the presence of the graphite in the compacted form; rather than in the nodular form. See the article by K. B. Palmer in British Cast Iron Research Association Journal, January, 1976, at pages 31-37.
Typical base metal compositions for the manufacture of nodular iron has the following composition:
Total Carbon: 3.20-4.10 percent by weight PA1 Silica: 1.5-2.80 percent by weight PA1 Manganese: Up to 0.80 percent by weight PA1 Phosphorus: 0.10 maximum percent by weight PA1 Sulfur: 0.03 maximum percent by weight PA1 Iron: Balance
The criticality of the sulfur content of the base iron has long been recognized. To obtain the maximum mechanical properties in sound nodular iron castings or sound compacted graphite castings, the sulfur content of the base iron composition should be less than about 0.03% at the time of inoculation. To retain the sulfur content at such a low level, extreme control measures have been taken. The sulfur can be controlled by using base materials or ingredients which are low in sulfur, or by desulfurizing the melt or by a combination of both.
Excessive sulfur in the base metal melt preferentially reacts with the magnesium or other nodular inoculant, and insufficient residual inoculant remains to perform its nodularizing or compacting function. Further, the presence of excess inoculant is harmful, since it results in inferior types of microstructures, the final nodular iron is more prone to carbidic structures, the inoculant itself is wasted, and attempts at obtaining a compacted graphite structure may result in an undesired nodular structure.
All in all, it will be readily appreciated that the control of sulfur is of extreme importance in the manufacture of either nodular iron or compacted graphitic iron and knowledge of the precise sulfur content is critical for efficient, metallurgically sound operation. Previously, the sulfur content has been determined by chemical analysis, by microstructure observation or by spectographic analysis. Each such test is time consuming and requires holding the base metal at pouring temperature while the test is performed, and each form of tests requires equipment and facilities which are not present in many foundries.
The only other test normally utilized is the visual fracture inspection performed on test pins poured from the iron composition after inoculation has been completed. Obviously, this test tells whether or not one has nodular iron or iron containing compacted graphite, but it does not reflect the character of the base metal itself, and it does not prevent the pouring of bad castings. The pouring of test pins prior to pouring the casting requires holding the inoculated molten metal during pouring and breaking of the test pins.
The problem of accurately determining the sulfur content is even more acute where the primary inoculant is added by the in-mold process, e.g. as proposed in U.S. Pat. Nos. 3,746,078 or 3,765,876. Here, the sulfur content must be known before pouring of the metal into the mold, since it is impossible to test the effectiveness of the inoculant prior to pouring.